Hello Elliott.
Thank you for helping me.
With your code it start it to work with button, but stop work for not .inline_popup a cases.
Please provide the code where this two situation can be united: $('.inline_popup a').magnificPopup and $('.inline_popup').magnificPopup

Here is how fuction.php (child) looks now

<?php
add_theme_support('avia_template_builder_custom_css');
function inline_popup_enabler(){
?>
<script>
(function($){
    $(window).load(function() {
      $('.inline_popup').magnificPopup({
        type:'inline',
        midClick: true 
      });
    });
})(jQuery);
</script>
function ava_custom_script(){
?>
<script>
(function($){
    $('.inline_popup a').addClass('inline_popup');
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'inline_popup_enabler');
add_action('wp_footer', 'ava_custom_script');

I’m glad that it helped :)
The main goal is to use Button Element with working Popup lightbox.
If it’s hard to set, i could use the code to build similar button element.

Could anyone help me with this problem, please?

Hello again.
Just pull the topic up. In last few days there are so many questions to you guys.
Thanks for your attention.

Hi this not exactly what i’m looking for.
If setup the custom Header size 150 px and choose: “Shrinking Headr” the menu will shrink to half size (150px / 2) after scrolling.
i would like to change this ratio, so after user scrolls the menu shrinks to 70% of origin size.